Marriott Platinum is not that high of an elite level when there is Ambassador and Titanium. If you are frequent guest at a certain property, the front desk manager may pre-upgrade you, assuming chances are low that they will sell those suites. Revenue management is more important to the hotels and if this is only your first time to the property, I would just wait until check in or ask nicely a day or two before your date of check-in your chances of a complementary upgrade to a suite.

As others have mentioned, suites upgrades are more common outside the US for higher level elites.